Astronomy Picture of the Day

Question Jovian planets are composed of fluids, which means: A) They are composed of a liquid interior layer and a gaseous outer layer. B) They have no rocky surface of any kind. C) They can exhibit convection. D) All of the above

Question What are the three processes which contribute to the banded structure observed on Jovian planets' surfaces ? A) convection, rapid rotation, differential rotation B) retrograde motion, convection, plate tectonics C) differential rotation,

Moons of Jovian Planets Jupiter alone has over 60, number discovered is growing due to better & better telescopes We will focus on the 6 largest: Diameter > 2,500 km & spherical There are also 12 “medium-sized” moons, massive enough to be spherical rock and water ice, which exhibit large amounts of cratering and approximately synchronous orbits. Most – no evidence of geological activity.  Many “small” moons, most probably captured interplanetary debris.  Why study Jovian moons?

Moons of Jovian Planets Why: Similarity to terrestrials, possibility of life

The Galilean Moons of Jupiter Closest to Jupiter Furthest from Jupiter (sizes to scale)‏ Europa - slightly smaller than our Moon Ganymede - largest moon in Solar System, slightly larger than Mercury Would you expect Jupiter to be hotter or cooler very early in its history? What might this imply for the Jupiter-Galilean moon system?

The Galilean Moons of Jupiter Closest to Jupiter Furthest from Jupiter (sizes to scale)‏ The closer to Jupiter, the higher the moon density (similar to behavior of terrestrial planets) => like mini “planetary” system. Intense heat of young Jupiter played role of sun.

Io's Vulcanism (Galileo Flyby of Io)‏ More than 80 active volcanoes have been observed. Can last months or years. Largest is bigger than Maryland - emits more energy than all Earth volcanoes combined! Most geologically active object in the solar system. Causes a thin atmosphere and smooth surface. Io is about the size of our moon. Why is it's volcanic activity surprising? plume volcano

Volcanic activity requires internal heat. Io is a small body. Should be cold and geologically dead by now. What is source of heat? Io and Europa are in a resonance orbit => Io stretched/squeezed => friction Start the clock now Europa Io Jupiter One Orbit of Io Europa Io Jupiter Two Orbits of Io Europa Io Jupiter

Europa may have Warm Ocean beneath Icy Surface (Jupiter's Moon Europa Video)‏ 860 km 42 km Icebergs? Dark deposits along cracks suggest “volcanic” eruptions of water with dust/rock mixed in. Fissures suggest large moving ice sheets.

What is source of heat? Similar to Io: resonance orbits with Ganymede and Io. Lack of cratering => process is ongoing Evidence of a warm subsurface ocean from measurements of a weak changing magnetic field. Warm ocean => life? Jupiter Io Europa Jupiter Ganymede

Ganymede and Callisto: Both heavily cratered Ganymede is differentiated but Callisto is not Ganymede shows evidence of ancient plate tectonics Neither show any obvious sign of recent geologic activity Unknown why two such similar bodies evolved differently!

Saturn's Large Moon – Titan – A moon with a thick atmosphere (lower temp than Jupiter's moons)‏ We will watch a video next class One of the main reasons Scientists find Titan so interesting is it may present an opportunity to study the kind of chemistry which occurred billions of years ago on Earth.

Neptune’s Triton Retrograde orbit → slowly spiralling towards Neptune due to location of tidal bulge and will be torn apart by gravity in 100 million years. Inclinded orbit + retrograde orbit → may be a captured Kuiper belt object. Currently geologically active → smooth icy, reflective surface.

Rings All the Jovian planets have a system of rings – but what are they made of ? Are they solid?

Saturn's Rings (all Jovians have ring systems)‏ - Inner radius 60,000 km, outer radius 300,000 km. Thickness ~100 m! - Composition: icy/rock/dust particles, 10m in diameter. Most a few cm “dirty snowballs” - A few rings and divisions distinguishable from Earth.

Origin of Saturn's Rings: If a large moon gets too close to Saturn, the tidal force breaks it apart into small pieces. The radius where this happens is called the Roche Limit (approximatly 2.5 x planet radius). Satellite must be held togther solely by its own gravity + must have similar density to planet for this to be an appropriate limit. Total mass of ring particles equivalent to moon 250 km in diameter. All ring systems in Solar System within or close to Roche Limit.

Voyager probes found that rings divide into 10,000's of ringlets. What could maintain this ringlet structure?

Gaps in Rings: Narrow gaps: Swept clean by small moonlets embedded within the rings. Moonlets are much larger than largest ring particles -> simply attract ring material as they orbit, leaving a less dense area. Cassini Division due to gravitational force of Saturn's innermost medium sized moon – particles are deflected into new orbits. (Ring Animation)‏

Shepherd Moons Some ringlets maintained by gravitational influence of “shepherd moons” Keep particles from straying outside the path of the ringlet Can cause complicated structure within the rings (i.e. braided ringlets)‏

Seasonal variations in our view of Saturn's Rings Over time Saturn’s rings change their appearance to terrestrial observers as the tilted ring plane orbits the Sun. The roughly true-color images (inset) span a period of several years from 1996 to 2000, showing how the rings change from our perspective, from almost edge-on to more nearly face-on.

Formation of Rings: Rings are young -> must either be newly formed or periodically replenished. Possible explanations: replacement by moon fragments chipped off by meteoric impacts and/or moon torn apart by tidal forces. Recall this will be the fate of Neptunes moon Triton.